The invention relates to a television transmitter for transmitting amplitude modulated signals, the transmitter comprising a kylstron amplifier, means for feeding the amplitude modulated video signal to the input cavity of the klystron, means for coupling the signal from the output cavity of the klystron to an antenna, and means for feeding the unmodulated video signal to a control electrode situated adjacent to the cathode of the klystron.
Such a transmitter is disclosed in U.S. Pat. No. 3,740,649 to Itoh in which it is stated that the efficiency and linearity is improved by the application of the unmodulated video signal to the control electrode.
A broadcast television signal normally comprises a carrier signal, an upper sideband having a bandwidth between 4 and 6 MHz and a lower sideband having a bandwidth between 0.75-1.25 MHz. The lower sideband is referred to as a vestigial sideband. The higher frequencies which are produced in the lower sideband by the modulator may be filtered either before or after application to the klystron amplifier. However, if the filter is situated at the output of the klystron amplifier, it has to be constructed so as to be able to dissipate considerable energy and consequently it is desirable to filter the modulated signal before application to the klystron amplifier. If, however, the video signal is now applied to the control electrode of the klystron the higher frequencies of the lower sideband may be reinserted by modulation of the beam current density.